8 8 ComForEn 2012 Keynote From Smart Grid to Universal Grid Generation, Storage and Communication in Hybrid Networks Wolfgang Gawlik, Institute of Energy Systems and Electrical Drives, Vienna University of Technology, Abstract Energy systems are hybrid networks, linking different forms of energy sources and loads. With growing amounts of renewable generation in the energy system, the hierarchical and centralized structure of the energy system gets increasingly transformed to a decentralized and distributed structure. The concept of Smart Grid is driven by these structural changes and provides solutions for the challenges resulting from the integration of distributed renewable energy sources into the system. The Universal Grid takes the concept of Smart Grid one step further as it exploits the fact that the hybrid networks of the future will be more closely interlinked. Communication is key to making these networks and concepts work, as it bridges the gap between different parts of the hybrid system. 1. Introduction For the search term Smart Grid, tens of millions of web pages are returned. For Universal Grid, the number is less than thirty thousand. These numbers indicate the necessity for more detailed definition and explanation: With Smart Grid, each of us tends to have their own definition with focus on a different aspect, and many might not have heard of Universal Grid before. Our energy systems are hybrid networks, consisting of parallel infrastructures for different forms of energy, e.g. electrical energy, gas and heat. At certain points in the system, these infrastructures are coupled, for example at a gas fired co-generation power plant producing both electrical energy and district heating. Certainly, the power plant is located in an area where sufficient gas supply is avail-

9 ComForEn able, electrical energy can be fed to the transmission system, and district heating can be efficiently used. However, in the traditional hierarchical energy system these coupling points are of significant size, but limited in numbers. As more and more renewable generation gets integrated into the energy system (renewable generation tends to be decentralized and comparably small in amounts, but larger in number of individual units), the traditional hierarchical structure is transformed into a distributed and decentralized setup and this is one of the main drivers for the Smart Grid. Basically, the Smart Grid developed from the application of automation and control features being used in the transmission network for decades to the distribution network. It is not that most of these applications weren t known of before; it just was not necessary or too costly to use them at the lower voltage levels, where sizes are smaller by three orders of magnitude but numbers are higher in the inverse ratio. However, this changes as the hierarchical structure with only few centralized power plants gradually gets replaced by a distributed structure with scores of decentralized power generation units. The Smart Grid provides the solution for challenges for the electrical energy system that are caused by this transformation. The Universal Grid extends this strategy and takes it even one step further by taking advantage of the fact that the coupling points for different energy sources are subject to the same transformation. Renewable energy sources tend to be more volatile and rather refined methods for prognosis of their generation pattern are available and yield acceptable results (Figure 1), but they cannot be controlled in the same way as thermal power units. Figure 1. Comparison of actual PV generation ( PV-Leistung gemessen ) and PV prognosis ( PV- Leistung berechnet ) taken from [1] Thus, storage will become increasingly important to balance out discrepancies between generation and load. Recent studies show that for a 100% renewable power generation scenario, existing storage capacity using conventional energy storage technology (i.e. pumped hydro) is way too small [1]. This means that new storage technologies, using conversion of energy carriers and creating additional coupling points between the different energy networks, are strongly required. In essence, Smart Grid is the solution to the distributed and decentralized renewable generation related to power, while Universal Grid provides answers in terms of energy. Communication is required for both concepts to enable and achieve optimum results.

10 10 ComForEn Power and energy An often seen trope is: A modern wind turbine with 2 MW rated power generates the electrical energy required for households or inhabitants. Thus, one single wind turbine can power a small village. This statement is not exactly true, but that happens if power and energy are mixed up. Power is the derivate of energy, i.e. energy per time. Energy is the integral of power, i.e. power multiplied by time. Consequently it is feasible that a modern wind turbine generates the same electrical energy over a certain time period, which is consumed by a certain number of households over that exactly same time period. But hardly ever will the power generation of the wind turbine exactly match the power consumption of these households. Most of the time, it will not even be close. However, our energy system works in such a way that power generation and load must be balanced all the time. Any larger deviation that is not balanced out within a few seconds results in a crash of the system, a.k.a. blackout. Thus, one single wind turbine alone cannot power a small village. Even if some part of the consumption may be managed ( demand side management ) in order to improve the balance, an additional part of the system has to take care of the rest. For a load flow calculation, this part is called slack. For the village demand and the wind turbine and probably some photovoltaic (PV) generation, the slack would be a rather big and fast generation and storage device of any sort. Figure 2 shows the requirement for such a storage device in a (fictional) setup of Austria being 100% powered by wind turbines and PV. Figure 2. p.u. daily load curves ( Last ) and wind generation, photovoltaic and required storage ( Speicher ) in a (fictional) Austria 100% powered with renewable energy sources. [2] Generally the rest of the energy system, including the thermal power plants, acts as slack. What happens is that, with an increasing number of volatile renewable energy sources, being a slack becomes increasingly difficult for the rest of the energy system [3]. When relying on the remaining thermal power plants becomes less of an option, any ideas for big and fast generation and storage devices become increasingly interesting and important. Storage of electrical energy itself is hardly possible. The electrical energy stored within the electrical energy system, e.g. in energized power lines or magnetized transformers, is negligible. The only significant quantity of energy stored within the system itself is given by the spinning masses of rotating electrical machines, i.e. generators and motors connected to the grid. All other energy storage relies on energy conversion, e.g. pumped hydro or power to gas. The Universal Grid, as defined in the previous chapter, features scores of these energy conversion points and thus the possibility for both short term and long term storage.

12 12 ComForEn 2012 It should be noted that this coupling is bi-directional. Hydrogen can be generated using electrolysis but also used to generate electricity in a fuel cell. Power to gas is a concept to synthesize methane using excess energy in the electricity network, which can then be stored for long time periods, and subsequently used to generate electrical energy in gas turbines again. The conversion technologies, their efficiency and related economic aspects are currently subject to immense research. An electricity network comprises two intrinsic communication channels: Frequency and voltage. Frequency is a global parameter, identical at all points of the system, synchronously coupled, and depending on the balance of load and generation within the entire electrical system. Voltage, on the other hand, is a local parameter, dependent on the conditions at a specific part of the system. However, both parameters are available at any point of the electrical energy system and thus offer a robust signal for any automation and control concept. This is why electrical engineers, especially protection specialists, are rather hesitant to base their backup concepts on any additional communication channel, which does not offer the same robustness and thus generally is only use for optimization. However, it is obvious that the Universal Grid, locally linking together the different parts of the hybrid energy system, does not feature such intrinsic communication channels. While each of the parts of the hybrid networks may offer a robust means for safe and secure automation and control, the Universal Grid itself will only work with communication channels bridging the gaps between different parts of this network. 4. Outlook Smart Grids are becoming reality. Today, the energy system is struggling with integrating more and more renewable energy sources. With more distributed generation coming into the networks, distributed storage and demand side management are getting increasingly important. Especially electric vehicles offer a huge potential for distributed storage and demand side management. In the near future, the Universal Grid, a hybrid network of electricity, hydrogen, gas and heat that is multidirectionally interlinked on a local level and merged via communication, will offer the storage and demand side management capabilities required for a sustainable energy system powered by renewable energy sources. References [1] TU Wien, ESEA/EA (Hrsg.): Super-4-Micro-Grid - Nachhaltige Energieversorgung im Klimawandel, approbierter Endbericht zum Forschungsprojekt im Rahmen der 1. AS Neue Energien 2020, Projektnummer: , Wien 2011 [2] Boxleitner, M. und Groiß, Chr Optimaler Erzeugungsmix für 100 Prozent regenerativen Strom in Österreich. 7. Internationale Energiewirtschaftstagung an der TU Wien : s.n., [3] Erneuerbare Energie braucht flexible Kraftwerke Szenarien bis Studie der Energietechnischen Gesellschaft im VDE (ETG), April 2012

14 14 ComForEn 2012 Co-Simulation of Power- and Communication-Networks for Low Voltage Smart Grid Control Friederich Kupzog, AIT Austrian Institute of Technology, Pavlos Dimitriou, Mario Faschang, TU Wien, Institut für Computertechnik Ralf Mosshammer, Siemens AG Österreich Matthias Stifter, Filip Andrén, AIT Austrian Institute of Technology Abstract The present passive operation in the electricity network is able to handle only limited amount of distributed generation. To avoid extension of grid capacity an intelligent infrastructure and smart grid control is proposed which will guarantee the compliance of limits given by EN For this reason, the different components of the grid have to communicate with the grid controlling unit to transfer real-time voltage measurements and commands. To assess the mutual influences between the electrical power grid on the one hand and the communication system on the other hand, a co-simulation architecture was developed. The methodology in which the components of communication and power simulation are coupled along with some first emulation results are presented in this paper. 1. Introduction The penetration of distribution networks with renewable sources or new network participators like electric vehicles is increasing [1]. For the integration of these elements in high density in the grid with also avoiding expensive extensions of grid capacity, intelligent infrastructure is needed which will also take care of keeping the voltage within the defined limits. That is one of the reasons why in the domain of the electrical distribution grids a paradigm shift can be noticed: distribution grid operation is heading in the direction of real-time information monitoring [2]. The project DG DemoNet Smart LV Grid searches for solutions for an active network operation at the low voltage level and tackles the increase of distributed generation (DG) hosting capacity in the low voltage grid. It is funded by the Austrian Climate and Energy Fund and was started in March Crucial for the project Smart LV Grid is to develop active voltage control methods and algorithms for the low voltage level because it is essential for the Distribution Network Operators (DNOs) to keep the voltage in between the predefined limits according to EN The present passive network operation is only able to handle limited amount of DG. The smart approach is to establish reliable communication between different network components (intelligent inverters at generation units, regulated tap changers from medium to low voltage or decentralized LV network controllers and further network components). The communication is required to transfer measured voltages

15 ComForEn from remote points in the grid to the voltage control components. For these control components to work in real or almost-real time, it is important that the communication will have known latencies and will have high reliability. To examine if it is suitable and if it follows the above requirements, the solution of the power line communication (PLC) is going to be tested. PLC does not require new communication cables but puts very strict restrictions on the available bandwidth. Therefore it is important to have analysis of mutual influences between the electrical power grid on one side and the communication system on the other side in simulations and emulations. Both domains have similar time constants so they cannot be analysed independently and thus the co-simulation is a very critical precondition for gaining useful research results (see chapter 3: Methodology of Co- Simulation). Based on this facts, an architecture for the coupled simulation is designed in the project. The scientific question that this co-simulation has to answer is: How do the electrical grid and the communication system influence each other and what are the constraints imposed by the PLC communication solution in the coordinated voltage control of distribution networks with high DG penetration. 2. State of the Art The experience of the DNOs shows that the key limiting factor for the development potential of DG in 30 kv networks and below is the overvoltage problem. Without massive grid reinforcement, additional generators would frequently be switched off by overvoltage protection [1]. Demand side management (DSM) and remote control of DG units in LV is in general not yet used in respect to voltage level. Such approaches are only implemented in pilot projects such as NetzQ in which data and control parameters are being transmitted over the power lines to control a distributed system of multiple solar inverters installed in a grid segment and thus increase power capability of the grid [9]. The characterization of low-voltage systems for the purpose of active control is relatively new. The challenge here is that in contrast to the medium voltage grid it can no longer be assumed that the three phases are equally loaded. For the project Smart LV Grid, four-wire models of the electrical network will be used. The line properties including the impedance of the neutral connector are measured by the Power Snapshot Analysis by Meters (PSSA-M) [2, 3]. For the communication simulation, the statistical nature of the PLC channel can be modeled from MAC measurement data, which is readily available. These simulations will be coupled in a Co-Simulation. In this simulation methodology the two subsystems will be coupled to achieve a simulation of the overall system [4]. Different Co-Simulators for electrical grid simulation and communication simulation have already been implemented. An example is the approach using integrated commercial off-the-shelf software in an EPOCHS architecture (Electric Power and Communication Synchronizing Simulator) [5]. Although its approach of using a commercial product for the electrical grid simulation has been embraced, there are arguments why this cannot be done with communication simulation: First the Smart LV Grid project does not use standard TCP protocols but rather the AMIS master-slave protocol (see chapter 4.2). In fact, this specific requirement for the use of the AMIS protocol is what makes our approach different from the other approaches. Second, the project assesses the development of different voltage control algorithms in a master-slave manner, which is

16 16 ComForEn 2012 a totally different approach from agents-in-different-control-components, presented in the EPOCHS architecture. 3. Methodology of Co-Simulation For the challenge to implement a pilot installation of smart low voltage grid operation in three different pilot regions in Austria, algorithms for active voltage control are developed within the project. These algorithms make use of active components, namely tap changers, energy management at converters for photovoltaic installations and electric car charging stations. In order to be able to verify the control algorithms before they are implemented in the field, an appropriate simulator has to be set up. Since remote measurements of network voltages play a major role in the voltage control algorithms, the properties of the communication channel(s) have to be taken into account in the evaluation of the control loop, if time horizons of control cycle, system reaction and communication delay are of the same magnitude. This is especially the case in low voltage power networks, where a significantly stronger volatility of energy flows can be observed compared to medium voltage grids, where a number of individual processes are cumulated. Therefore, a coupled simulation of the power grid on one hand and the communication flow on the other is necessary. Since this is done in the approach presented here with different simulation tools and more than one integration layer, the approach is a co-simulation [4]. It is intended to use the co-simulator not only for simultaneous simulation of power and communication system as described above, but also to implement mutual influences of both systems into the simulator. While the power line communication system has only minor effects on the power system, the power system will strongly influence the communication performance especially in case of tap changing. Also the communication noise caused by the inverters might be critical. These properties however will first be evaluated in laboratory tests, before they will be added as features to the simulator. 4. Co-Simulation In general, the coupling of communication and electrical simulators can serve various purposes, dictated by the abstraction level of the simulation. For example, the time-variant dynamics of the grid could be applied to a low-level physical simulation of the DLC simulation to assess the impact of load flow on communication characteristics. The described co-simulation does not describe the dynamic processes of PLC communication as it is only event-based modelled. The time-cycles of the simulation at hand are at a much greater magnitude, however. The goal is to gauge the impact of communication delays on control algorithms. Co-simulation is necessary to have a realistic impression of the behavior of (electrical) network components, which in turn determine the time constants of the control loop. In the ideal case, all components of the simulated system can be integrated into a single tool. However, due to different requirements, such tools are mostly not available, and specialized simulation components have to be coupled via defined interfaces.

17 ComForEn Architecture To integrate the heterogeneous simulation components into a coherent simulation framework, a stand-alone, minimalistic server component ( adapter ) was developed to efficiently handle data traffic between simulation clients. The adapter decouples up- and downlink connections between two or more clients with an internal data model, which is loosely based on a subset of IEC61850, which is a standard for electrical substation automation. The benefit of this decoupling lies in the option to allow communication between clients with different application-layer protocols. The only prerequisite for a client to take part in the simulation is that its application layer dialect must have a direct mapping to the IEC61850 data model. Messages received by the adapter are processed through a translation layer, handed to the internal model, and sent out, optionally after another translation iteration, to the receiving client. Figure 1. Architecture of the Co-Simulation system To build an emulation chain, the grid simulation, communication model and grid control are linked via two separate adapter components with bidirectional communication channels like depicted in Figure 1. Simulation flow is top-down, with commands from the controller reaching the grid simulation after experiencing a packet delay induced by the communication simulator. Reply packets from the grid simulation are handled in a similar fashion. Simulation additionally adds the event queue and synchronizer to the chain via a third adapter. The synchronizer does not easily bind to a socket, but this can be handled with the adapter itself. The transport layer is not fixed to be a TCP/IP socket, but can be an arbitrary component with a compatible interface towards the adapter. Together with the translation layer, this architecture allows for a wide range of options for the connection of external simulation components.

18 18 ComForEn Communication Simulation As mentioned above, all communication in the project is done via power line carrier (PLC), especially via the Automated Metering and Information System (AMIS) PLC system introduced by SIEMENS [6]. As the communication system is a master/slave system, all communication has to be initiated by the data concentrator (DC) which acts as the communication master. Figure 2: Functional block diagram of the sending unit [6] Data transmission in the AMIS system follows the principle described in section 4.1. The functional block diagram of the data transmission unit in the sender is depicted in Figure 2. Transmission on the physical layer of AMIS PLC is based on fast frequency hopping spread spectrum technology in CENELEC A-Band between 3 and 95 khz. All data is modulated either by differential binary phase shift keying (DBPSK) or differential quadrature phase shift keying (DQPSK) depending on the selected transmission mode. To avoid bit errors during the transmission, all data coming from the MAC layer is coded using bitwise repeating block codes. After this coding process the resulting bitstream will be segmented into blocks. Theses blocks then will be interleaved before data is forwarded to the previously mentioned spread spectrum transmission [6]. As mentioned before, all communication is initiated by the master device which is the AMIS data concentrator (AMIS-DC). Requests from the AMIS-DC are sent to the slaves (AMIS-meters, AMIS-load switches, etc.) with a specific number of retransmissions defined by the master. Every node which receives such a message repeates it simultanously as long as the predefined number of retransmissions is reached, event if the packet is not destined for that node. The same system is used by the nodes when sending the answer packet to the DC. These simultanous retransmissions create hop layers depicted in Figure 3. The benefit of simultanous packet retransmission is, that nodes which cannot be directly adressed by the master are still reachable due to the intermediate nodes which are acting as repeaters [6]. Figure 3: Hop-Layers due to simultaneous message repeating In the example in Figure 3 several nodes are connected via power line communication to the DC. Node 1 and node 2 can be directly reached from the master without any retransmission. All other nodes need intermediate nodes in between repeating the packets to be able to communicate with the master. For example node 6 needs at least two retransmissions of the packets to be reachable by the master. Even if the destined node receives its packet in the first retransmission phase, it has to wait

19 ComForEn until the predefined number of retransmissions is reached before sending an answer packet to the master. Due to mutual influence of the electrical network and the environment to the power line communication packets may be lost or distorted during their transmission. To overcome the negative influences and thereby caused communication problems the previously mentioned repeating communication algorithm was introduced. By analysing communication logs of several AMIS-DC a statistical description of the AMIS communication system is possible. There are two parameters which are used for the statistical description of the communication channel: First, the loss probability P loss and, second the approximately Gaussian distributed delay time T delay. The values for those two characteristic parameters were determined by analysis of about two million AMIS log records. The analysed records have been created by AMIS DCs in typical Austrian low voltage distribution grids. The obtained data for P loss and T delay is very useful for the communication simulation in the ComSim part of the simulator, described below. All messages in the simulator are passing the ComSim where the values for P loss and T delay are set according to the number of retransmissions (hops) defined by the grid control unit. a) b) Figure 4: Architecture of the Communication Simulator in a) Emulation Mode and b) Simulation Mode The architecture of the communication simulator ComSim is depicted in Figure 4. The ComSim module consists of four independently running threads. Three of them act as ports towards the other modules of the co-simulator, which are the grid control, the power grid simulator and the simulation control module. The fourth thread is the AMIS channel model itselve, which is responsible for modelling the AMIS PLC communication. The prviously mentioned simulation control is only necessary in simulation mode and is responsible for the syncronization of all cosimulation units. Concerning the data flow there must be a distinction between the two different operation modes: a) Emulation mode and b) Simulation mode. In the emulation mode all communication takes place between the grid control and the power grid simulation via the AMIS channel model. As shown in Figure 4 a) simulation control is not required in this operation mode. All transmitted packets have to pass the AMIS channel model where the

20 20 ComForEn 2012 particular loss probability and the packet delay time are determined according to the packets retransmission (hop) count. If the packet gets lost it will not be forwarded but delayed for the determined amount of time. Figure 4 b) shows the communication simulator in simulation mode. Packet which are sent by the grid control unit or the power grid simulator still have to pass the AMIS channel model where the loss probability and the packet delay time are determined. This information is added to the packet which then is forwarded to the simulation controller without intentionally delaying it. Delaying and reordering of the packets takes place in an event queue in the simulation control. All packets coming back from the simulation control are then forwarded to its destination. 4.3 Power Grid Simulation For the numerical simulation of distribution network, PowerFactory is used, which is capable of performing steady state and transient analysis. In the transient simulation (so called RMS in power factory) the simulation can be synchronized to (scaled) real time. This is necessary to run the emulation of the analysis of the system under investigation. In simulation mode the power flow analysis can be be time synchronized with the simulation control via several mechanisms. Different possibilities to interface with the co-simulation environment have been evaluated and implemented. Figure 5 shows the implemented interfaces via OPC (1), RCOM (2) and TCP/IP Sockets. Simulation Infrustructure Figure 5: Interfaces for the power grid simulation for transient or steady state analysis The low voltage transformer model type is extended with the on load tap changer (OLTC) capability, including the number of steps and the delta voltage per step. According to the control scheme, the tap position or the internal transformer controller voltage set value are set, using the power factory programming. A inverter controller model has been implemented in Power Factory, following a dq/du droop curve, according to the given characteristic, which can be set during the simulation from the control algorithm. A full inverter model, including the power electronics is available in Matlab/Simulink and can be interfaced via DSL blocks during transient simulation. Electric vehicles with charge control respective their charging stations can be connected to loads in Power Factory via various interfaces like OPC. The simulation of the energy demand of the single electric vehicles can be based on simple traffic models and event lists like commuting from home to work or shopping sites.

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